The present invention relates to a continuous-wave ultraviolet laser light generating apparatus suitable for use in a continuous-wave ultraviolet laser light generating apparatus used as a light source used when a material is processed or worked by photolithography.
FIG. 1 is a schematic diagram showing an arrangement of a continuous-wave ultraviolet laser light generating apparatus. A continuous-wave ultraviolet laser light generating apparatus has been proposed, which, as shown in FIG. 1, has a laser light source unit 1 for generating green laser light, for example, a wavelength converting apparatus 4 that includes an optical resonator, i,e., a so-called external optical resonator 2 and a wavelength convertor 3 disposed therein and converts a wavelength of the laser light emitted from the laser light source unit 1 to derive continuous-wave ultraviolet laser light L.sub.UL, a locking circuit 5 for locking a cavity length of the optical resonator 2 at a predetermined cavity length, and an electrooptic phase modulator 6 used for the locking.
In this case, the laser light source unit 1 employs, for example, a continuous-wave Nd: yttrium aluminum garnet (YAG) laser (not shown) as a light source. A second harmonic generator (SHG) (not shown) at the first stage derives green laser light with a wavelength of 532 nm from the laser light from the light source.
The green laser light with a wavelength of 532 nm, for example, emitted from the laser light source unit 1 is converted by a SHG at the second stage composing the wavelength convertor 3 in the above wavelength converting apparatus 4 to obtain the continuous-wave ultraviolet laser light as a forth harmonic of the above YAG laser.
The optical resonator 2 is formed of at least one pair of mirrors, e.g., four mirrors M.sub.1 to M.sub.4 as shown in FIG. 1. The wavelength convertor 3 is disposed in an optical path formed by the four mirrors M.sub.1 to M.sub.4.
The wavelength convertor 3 is formed of a nonlinear optical crystal made of BBO (.beta.-BaB.sub.2 O.sub.4), for example.
The above-mentioned optical resonator 2 is provided in the continuous-wave ultraviolet laser light generating apparatus in order to effectively and stably generate light having a predetermined wavelength, i.e., ultraviolet laser light. By Drever locking method, a resonance frequency of the optical resonator 2 is set equal to a frequency of the green laser light, for example, emitted from the laser light source unit 1 and introduced into the optical resonator 2, thereby the green laser light being effectively introduced into the optical resonator 2. This method allows stable generation of the ultraviolet laser light L.sub.UL having a desired wavelength, e.g., 266 nm (see Japanese laid-open patent publication No. 53593/1994).
In this case, for example, one mirror M.sub.1 composing the optical resonator 2 is attached to an electromagnetic positioning device, i.e., a voice coil motor (VCM) 7. The VCM 7 is driven to finely move the mirror M.sub.1, thereby an optical path length in the optical resonator 2 being finely changed. Specifically, the locking circuit 5 controls the VCM 7, i.e., controls the position of the mirror M.sub.1, so that it is possible to control the resonance frequency fr. Thus, only light having a certain frequency is introduced into the optical resonator 2 and any light having other frequencies is reflected.
FIG. 2 shows reflectivity characteristics of the optical resonator 2. A horizontal axis depicts a frequency of incident light, and a vertical axis depicts a reflectivity of the optical resonator 2. As shown in FIG. 2, the reflectivity with respect to the light having the resonance frequency fr is 0%, and the reflectivity with respect to other light is about 100%. The resonance frequency fr is determined by an optical path in an external resonator. A width .DELTA.fr of the frequency of the light which can be transmitted through a mirror is determined by reflectivity of each mirror.
On the other hand, as shown in FIG. 3, a frequency f.sub.L of the incident laser light introduced from the laser light source unit 1 into the optical resonator 2 is distributed over a certain width .DELTA.f.sub.L. However, the width .DELTA.f.sub.L is considerably small compared with the width .DELTA.fr.
In an arrangement employing the Drever locking method, a photodetector 8 such as a photodiode or the like for detecting light reflected by the optical resonator 2 is provided as shown in FIG. 1. An intensity of the reflected light is obtained by multiplication of the reflectivity of the optical resonator 2 shown in FIG. 2 and the laser light frequency shown in FIG. 3. It is assumed that the resonance frequency fr is changed as shown in FIG. 4A by moving the mirror M.sub.1 from a position A to a position E, for example. At this time, the reflected-light intensity is changed as shown in FIG. 4C. If the mirror M.sub.1 is fixed to a position where the reflected-light intensity is 0 (a position C shown in FIG. 4C), then the resonance frequency fr and the laser light frequency become coincident with each other, thereby the incident laser light being introduced into the optical resonator 2.
As described above, however, since the frequency of the incident laser light is fluctuated in fact from time to time, it is necessary to constantly adjust the position of the mirror M.sub.1 by following detected difference between the laser frequency f.sub.L and the resonating frequency fr. To this end, it is not sufficient to only detect the above-mentioned position where the reflected-light intensity is 0, because it is impossible to determine whether the difference between the laser frequency f.sub.L and the resonance frequency fr is caused by a relationship of fr&lt;f.sub.L (obtained at a mirror position B shown in FIG. 4C) or fr&gt;f.sub.L (obtained at a mirror position D shown in FIG. 4C), only by detecting the reflected-light intensity (in other words, an even function whose center is a point satisfying fr=f.sub.L is established in the graph of FIG. 4C).
It is necessary that an error signal whose polarity is inverted depending upon fr&gt;f.sub.L or fr&lt;f.sub.L is generated and a servo control to tune the resonance frequency fr to the laser frequency f.sub.L is effected based on the error signal. To this end, the phase modulator 6 shown in FIG. 1 is provided to modulate a phase of the laser light for generating sidebands of the laser-light frequency. When the phase modulation is carried out at a frequency fm, the sidebands are generated at frequencies f.sub.L .+-.fm in addition to the laser frequency f.sub.L as shown in FIG. 5.
Subsequently, a principle of generating the error signal from the reflected light obtained when the laser light having the generated side bands is incident on the optical resonator 2 and reflected thereby will be described below with reference to FIGS. 6A to 6F which are diagrams showing spectra of the reflected-light intensity.
The reflected light from the optical resonator 2 has three components of frequencies of f.sub.L -fm, f.sub.L, and f.sub.L +fm as shown in FIGS. 6D to 6F. When the reflected light is detected by the photodetector 8, the signal currents having the following three components are measured:
(1) a DC reflected-light intensity component I.sub.DC ; PA1 (2) a beat signal (an amplitude A) I.sub.AC1 with the frequency fm obtained from the difference between the frequencies of the light having the frequency f.sub.L and the light having the frequency f.sub.L -fm, i.e., EQU I.sub.AC1 =A exp [-i2.pi.fm t]; PA1 (3) a beat signal (an amplitude B) I.sub.AC2 with the frequency fm obtained from the difference between the frequencies of the light having the frequency f.sub.L and the light having the frequency f.sub.L +fm, i.e., EQU I.sub.AC2 =B exp [i2.pi.fm t]. PA1 (1) an amount of light with a wavelength of 532 nm which the crystal absorbs small; PA1 (2) an optical damage of the crystal is prevented from being generated by a high-power laser light with a wavelength of 532 nm, specifically, the crystal can endure the laser light with an intensity of about 10 W; and PA1 (3) the crystal's figure Q of merit for the electrooptic phase modulation is high and hence the crystal can be driven at a low voltage. PA1 (1) The figure of merit required for the phase modulator is small so that a high drive voltage is required. The figures of merit of KDP and ADP are 34 pm/V and 27 pm/V, respectively. If the crystal length l=12 mm and the distance d between electrodes d=3 mm, it is necessary to respectively apply the AC voltages with the amplitudes V.sub.o of 125 V and 157 V to the KDP and the ADP for realizing .beta.=0.1. The phase modulation frequency .OMEGA. of the continuous-wave ultraviolet laser is about .OMEGA.=10 Mhz. A complicated high-frequency amplifier circuit is required to obtain such high-frequency and large-amplitude signal voltage, which increases production costs of the phase modulator and the overall size of the apparatus and leads to disadvantage in practical use. PA1 (2) Since the phase modulator formed of KDP or ADP requires a high voltage as described above, it is difficult to effect the digital modulation, the analog modulation and the APC of the ultraviolet laser light by modulating the phase modulation depth .beta.. PA1 (3) Moreover, since the KDP and ADP are chemically unstable and highly deliquescent, they are chemically changed by their absorption of moisture from the air. Therefore, when optical crystals made of the KDP and the ADP are used, they are immersed in oil or enclosed in nitrogen gas and further they must be sealed by shielding them with a complete shielding body in order to shield them from the outside airtightly. However, if the optical crystal is completely sealed by the shielding body and further the shielding body is provided with windows for incident laser light and emitted laser light, then there is not only the problems of complicated manufacturing process, expensive costs and unsatisfactory reliability but also the problem that a material for the windows and the oil used for oil immersion reduce amounts of the incident light and the emitted light.
and
If the mirror M.sub.1 is moved to change the resonance frequency of the optical resonator 2 around the laser frequency as shown in FIGS. 6A to 6C, then the spectra of the reflected light shown in FIGS. 6A to 6C are changed as shown in FIGS. 6D to 6F, respectively. Specifically, an intensity relationship between the light having the frequency f.sub.L -fm and the light having the frequency f.sub.L +fm is reversed at the frequency at which the resonance frequency is coincident with the laser frequency. This fact reveals that an intensity relationship between the amplitudes A and B of the two beat signals I.sub.AC1 and I.sub.AC2 is also reversed around a resonance point. A position where the intensity relationship between the amplitudes A and B is reversed is detected and the mirror M.sub.1 is set at the position, thereby the resonance frequency and the laser frequency becoming coincident with each other.
When it is determined which is larger, the amplitude A or the amplitude B, a phase delay amount of an AC signal I.sub.AC from the photodetector 8 is utilized. The AC signal I.sub.AC is obtained by linear combination of the beat signals I.sub.AC1 and I.sub.AC2 and has a phase delay amount .phi. in response to a ratio of the amplitude A to the amplitude B. ##EQU1##
Since values of the phase delay amount .phi. can be represented by an odd function whose center is the point satisfying fr=f.sub.L, the phase delay amount .phi. can be employed as the error signal. Specifically, the phase delay amount .phi. is detected, and the servo control is effected on the VCM 7 for controlling the position of the mirror M.sub.1 based on the phase delay amount .phi., i.e., the error signal. Thus, a cavity length, i.e., the resonance frequency of the optical resonator 2 is made coincident with the frequency of the incident laser light. Accordingly, the laser light is effectively introduced into the optical resonator 2. FIG. 7 shows an example of the error signal (which is represented by a curve 20 in FIG. 7).
As described above, when the continuous-wave. ultraviolet laser light generating apparatus employs the Drever locking method, it is possible to stably and effectively generate the desired ultraviolet laser light. However, when the continuous-wave ultraviolet laser light generating apparatus is practically fabricated, there are various problems.
When the Drever locking method is employed as described above, the electrooptic phase modulator 6 is employed as shown in FIG. 1. Specifically, the phase modulator 6 is made of an electrooptic crystal presenting an electrooptic effect in which a refractive index thereof is changed depending upon an applied voltage. When an AC voltage having a frequency .OMEGA. is applied to the electrooptic crystal and laser light is made incident on the electrooptic crystal, a phase of transmitted light is modulated in a sine fashion. For effecting the above-mentioned Drever locking, phase modulation with phase modulation depth .beta. of about 0.1 is effected on incident green light. The phase modulation depth .beta. depicts an amplitude of phase modulation obtained when a sine phase modulation is effected. An electric field E.sub.pm of the laser light modulated in the above manner is represented by the following equation; EQU E.sub.pm =E.sub.o exp j{.omega.t+.beta. sin .OMEGA.t}
where E.sub.o depicts a field amplitude of the laser light, .omega. depicts the frequency of the incident laser light, and .OMEGA. depicts a modulation frequency. A magnitude of the phase modulation depth .beta. is given by a figure Q of merit of the electrooptic phase modulation of a crystal, a length l of the crystal, a distance d between electrodes, and an amplitude V.sub.o of an AC voltage applied to the crystal, being represented by the following equation; EQU .beta.=(.pi.Qv.sub.o /.lambda.)(1/d)
where .lambda. depicts a wavelength of incident light.
On the other hand, it is possible to modulate an intensity of the ultraviolet laser light L.sub.UL by modulating the magnitude of the phase modulation depth .beta.. When .beta.=0.1, the ultraviolet laser light intensity is almost maximum. As the phase modulation depth .beta. is increased, the ultraviolet laser light intensity is decreased. When .beta.=2.4, the ultraviolet laser light intensity becomes almost 0. Specifically, it is possible to modulate the ultraviolet laser light L.sub.UL in a digital or analog fashion by modulating an amplitude of a high-frequency signal applied to an optical phase modulator.
It is possible to effect automatic power control (APC) by monitoring the ultraviolet laser light intensity and feeding it back to the phase modulator 6. FIGS. 8A, 8B are schematic diagrams showing digital modulation using the modulation of the phase modulation depth .beta.. FIG. 8A shows a signal voltage which is applied to the phase modulator and obtained by modulating an amplitude of a carrier signal of a frequency .OMEGA.. FIG. 8B shows an ultraviolet laser light intensity obtained at this time.
The electrooptic crystal composing the electrooptic phase modulator for use in the Drever locking in the above-mentioned continuous-wave ultraviolet laser light generating apparatus is selected from those satisfying the following conditions:
Crystals for use in the phase modulation of visible light are roughly divided into two groups: one group includes systems of lithium tantalate (LiTaO.sub.3) (hereinafter referred to as LT) and lithium niobate (LiNbO.sub.3) (hereinafter referred to as NT); and the other group includes systems of potassium dihydrogenphosphate (KH.sub.2 PO.sub.4) (hereinafter referred to as KDP) and ammonium dihydrogenphosphate (NH.sub.4 H.sub.2 PO.sub.4) (hereinafter referred to as ADP). Crystals of the LT and NT systems have high figures of merit for the electrooptic phase modulation but have low durability against optical damage. Therefore, it is impossible to employ these crystals in the continuous-wave ultraviolet laser employing the high-power laser light.
Accordingly, the phase modulator of this kind employs the electrooptic crystal made of KDP or ADP.
However, the phase modulator made of the electrooptic crystal of KDP or ADP is encountered by the following problems.
On the other hand, if the crystal length is increased for reducing the drive voltage, there is then the problem that production costs of the crystal are increased and the phase modulator becomes large-sized.
If the distance d between the electrodes is decreased for reducing the drive voltage, there is then the problem that an incident aperture of the phase modulator for the incident laser light becomes small to thereby reduce a diameter of the laser beam which can be incident on the phase modulator.
As described above, it is difficult to drastically reduce the drive voltage, and the complicated, expensive high-frequency amplifier circuit is required.
For stopping emission of the ultraviolet laser light (by setting .beta.=2.4), a high-frequency voltage having an amplitude of about 3 kV is required.